The preparation of solution-processable graphene from graphite oxide typically involves a hydrazine reduction step, but the use of such a reagent in the large-scale implementation of this approach is not desirable due to its high toxicity. Here, we compare the deoxygenation efficiency of graphene oxide suspensions by different reductants (sodium borohydride, pyrogallol, and vitamin C, in addition to hydrazine), as well as by heating the suspensions under alkaline conditions. In almost all cases, the degree of reduction attainable and the subsequent restoration of relevant properties (e.g., electrical conductivity) lag significantly behind those achieved with hydrazine. Only vitamin C is found to yield highly reduced suspensions in a way comparable to those provided by hydrazine. Stable suspensions of vitamin C-reduced graphene oxide can be prepared not only in water but also in common organic solvents, such as N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP). These results open the perspective of replacing hydrazine in the reduction of graphene oxide suspensions by an innocuous and safe reductant of similar efficacy, thus facilitating the use of graphene-based materials for large-scale applications.

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Graphene has attracted a great deal of scientific interest in latter years owing to its unique properties, with many prospective applications being actively investigated at present. However, the actual implementation of graphene in technological uses will depend critically on the development of appropriate methodologies for its mass production. In this regard, one of the most promising approaches is based on the exfoliation and reduction of graphite oxide. Graphenes derived from graphite oxide can be prepared at low cost and high throughput, can be further processed in a number of solvents, and are chemically versatile, among other attractive features. In an environment-conscious world, the availability of green approaches toward graphene production would also constitute an added advantage. During the last year, different environmentally friendly methods for the production of graphene from graphite oxide have emerged, which we highlight here. These are based on solvothermal and electrochemical processes, as well as on the use of green reductants. Several open questions and possible future directions for this research topic are also discussed. L: GuardiaLaura Guardia graduated in Chemistry from University of Buenos Aires (Argentina) and received a PhD degree from the University of Oviedo on molecular recognition of antibiotics. Currently, she is a post-doctoral researcher at INCAR, working on the synthesis and applications of graphene.

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Chemical reduction of exfoliated graphite oxide (graphene oxide) has become one of the most promising routes for the mass production of graphene sheets. Nonetheless, the material obtained by this method exhibits considerable structural disorder and residual oxygen groups, and reports on their microscopic structure are quite scarce. We have investigated the structure and chemistry of graphene oxide samples reduced to different degrees using atomic force and scanning tunneling microscopy (AFM/STM) as well as X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), respectively. TPD and XPS results indicate that reduction proceeds mainly by eliminating the most labile oxygen groups, which are ascribed to epoxides and hydroxyls on basal positions of the graphene plane. AFM/STM shows that the sheets are composed of buckled oxidized regions intermingled with flatter, non-oxidized ones, with the relative area of the latter increasing with the reduction degree.

The oxidation and etching of reduced graphene oxide (RGO) by thermal oxidation in air, microwave oxygen plasma, ultraviolet-generated ozone and scanning tunneling microscopy (STM) lithography have been investigated. This type of graphene exhibited a higher reactivity towards oxidation than that of pristine graphite (taken as a reference carbon material), which could be related to its intrinsically defective structure. Etching of RGO as a result of thermal oxidation in air was started at as low a temperature as 400 °C, compared with 500 °C for graphite, indicating that the defects present on the former are different in nature to those found on the surface of pristine graphite. The morphological evolution of individual RGO sheets upon oxidative attack was consistent with the sheets being essentially a patchwork of minute domains (a few to several nanometers large) with two distinct reactivities, higher (lower) reactivity associated to defective (defect-free) domains. The introduction of oxygen functional groups on the basal plane of RGO was revealed directly by X-ray photoelectron spectroscopy and indirectly through STM. STM lithography enabled discrimination between oxidation proper (introduction of oxygen groups) and etching (desorption of the groups as CO or CO 2) of the RGO sheets through control of the applied bias voltage. The former was visualized in the STM images as locally depressed features of electronic origin, whereas etching led to the generation of actual trenches on the sheets. Taken together, the

The implementation of green approaches towards the preparation of graphene and graphene-based materials with enhanced functionality from graphite oxide has been relatively little explored. Particularly, the use of bioreductants and the testing of their relative efficacies is an incipient area of research. Here, a pool of 20 environmentally friendly, natural antioxidants has been tested for their ability to reduce graphene oxide. These antioxidants were mostly vitamins, amino acids and organic acids. By establishing a protocol to systematically compare and optimize their performance, several new efficient bioreductants of graphene oxide have been identified, namely, pyridoxine and pyridoxamine (vitamin B 6), riboflavin (vitamin B 2), as well as the amino acids arginine, histidine and tryptophan. These biomolecules were used to prepare reduced graphene oxide-silver nanoparticle hybrids that displayed colloidal stability in water in the absence of additional dispersants. Particularly, hybrids prepared with pyridoxamine exhibited a combination of long-term colloidal stability and exceptionally

A green, photochemical approach for the liquid-phase synthesis of carbon nanomaterialsupported silver nanoparticles is proposed. The method is based on irradiating a colloidal dispersion containing the carbon nanomaterial, a metal precursor and an environmentally friendly reducing agent (bioreductant) with UV light at room temperature. Two representative carbon materials have been used, namely, platelet-type graphite nanofibers and graphene oxide. The experimental conditions and possible mechanisms that afford the photochemical growth of the nanoparticles on each carbon support are also investigated and discussed. In addition, the resulting carbon-silver nanoparticle hybrids are demonstrated to be notably effective catalysts for the reduction of 4-nitrophenol to 4-aminophenol with NaBH 4 . Particularly, the graphene oxide-based samples were seen to exhibit exceptional catalytic activity towards such reaction. Finally, it is also shown that with a proper choice of bioreductant the present UV approach can afford highly reduced graphene oxide samples comparable to those attained with well-known, efficient chemical reductants (e. g., hydrazine at ~100 ºC), thus constituting an attractive room temperature alternative to such reduction methods.